Four areas of investigation will contribute to the objective of elucidating the mechanism and regulation of intestinal calcium transport: 1) Investigate the mechanisms of calcium absorption in rat cecum as modulated by vitamin D, calmodulin, and calcium binding proteins. 2) Using the above preparation, we will establish the site of action of ATPase inhibitors and neuroleptics as well as vitamin D3 and sodium on calcium absorption. 3) Define the interaction between calcium binding and translocation as modulated by vitamin D at the microvillus membrane of both small intestine and cecum. 4) Establish the mechanisms for calcium efflux across the basolateral membrane of intact enterocytes by utilizing isolated basolateral vesicles to define the relative roles of Na:Ca exchange and Ca-ATPase in regulated calcium absorption.